Despite recent rapid advances in communication technology, communications in underground mines have remained relatively crude and simple. In view of the environment and the possible dangers, there is a strong demand for a satisfactory communication system, but nonetheless mine communications are presently far from adequate.
Various communication systems have been proposed, based on radio communication. However, particularly where the mine is cut in a conducting ore body, there is no reasonable electromagnetic wave propagation through the ore body, and hence the only propagation that can be used is along individual tunnels, etc. Thus, the propagation of radio waves is quite different from surface operation. VHF radio equipment has been used in mines, but at best one can only obtain a range of a few hundred meters. If the transmitted power is increased, this does not have a great deal of effect on the range. Furthermore, it has to be borne in mind that in many mines, the transmitted power is severely limited. For example, because of the possible danger of accidentally setting off blasting caps, in many mines, the transmitted power is limited to one watt.
If higher transmission frequencies, for example in the UHF range are used, this may give improved range, but this is entirely on a line of sight basis. Any corner or even a slight bend in the tunnel becomes a complete barrier to transmission.
At very low frequencies, the order of a few hundred KHz, it is possible to obtain a form of radio communication, but this form of transmission has some serious limitations for personal communication systems because of the size of portable antenna required and the limited bandwidth available.
Accordingly, a solution that has been evolved to these transmission problems is the use of so-called "leaky feeders" or "leaky coax cable". A "leaky coax cable" is a special type of co-axial cable that has the property of carrying a radio signal over a considerable distance, but at the same time allowing a certain, designed amount of the signal to leak out along its length, so as to provide communication with radio sets in the vicinity. Here, the relevant vicinity is the cross section of the tunnel through which the cable passes. Thus, the distance over which the leaky coax cable has to transmit is never more than the width or diameter of the tunnel. It should also be appreciated that the reference to "leakage" is intended to cover both transmission from the cable, and also reception by the cable of a signal from an adjacent transmitter. The leaky coax cable can then be connected to a conventional base station which acts as the main receiver/transmitter. In this specification, including the claims, the term "leaky coax" or "leaky coax cable" denotes a co-axial cable having an outer conductive screen that is provided with openings or imperfections such that the leaky coax cable is capable of both receiving signals from and transmitting signals to the immediate vicinity of the cable, and it also encompasses any elongate antenna which has similar properties and can be configured to any desired shape. The openings could be provided by an open braid or spiral wrap screen, or by other openings in the outer shield of the cable.
Leaky coax cables have some limitations. By their very nature, if larger areas are to be covered, the signal transmitted along them has to be reinforced or amplified regularly. In current leaky coax cable systems, in-line amplifiers are inserted in the leaky coax cable at appropriate intervals, with power being supplied through the leaky coax cable itself. The amplifiers can then be spaced relatively close together, e.g. in the order of a few hundred meters, so that a relatively uniform signal strength can be maintained throughout a lengthy installation. This in turn enables the characteristics of the leaky coax cable to be relaxed, and relatively low efficiencies can be tolerated. It also enables the original signal injected into the leaky coax cable to be of a low level, meeting relevant mine safety standards.
Various types of leaky coax cable have been developed. One design consisted of a conventional co-axial cable, with portions of the external sheath removed. More recent configurations have a specially configured external sheath that only covers part of the exterior, so as to provide a controlled amount of leakage along its length.
Now, as any radio communication system usually requires two-way communication, this raises the problem of providing two-way amplification of the signals travelling in the two directions along the leaky coax cable. A variety of techniques have been developed to deal with this problem. In one, the system is essentially configured into a complete circle, so that all signals are always travelling in the same direction around the circuit, and eventually the signals arrives at their desired locations. This avoids the problem of having signals travelling in two directions.
Another technique that has been used is to transmit the two signals along the leaky coax cable at quite distinctly separate frequencies. Thus, in a so-called FD4 system, the signal frequency is divided by four at one end of the leaky coax cable and transmitted back along it. At the other end, the signal is multiplied by four before being transmitted back. This assures that there is a factor of four between the frequencies of the signals travelling in different directions. It is then a relatively simple matter to provide in-line amplifiers for signals travelling in the two directions, with the amplifiers having appropriate filters to ensure that they only amplify the desired signal. Such a system has been installed in a variety of locations with reasonable success.
A fundamental draw back and limitation to all these systems is that they essentially provide for one channel of communications, e.g. for a single frequency in each direction. Whilst suggestions have been made that a wide range of standard facilities and attachments can become available for data transmission and that the system can be extended beyond the surface of the mine, these systems fundamentally provide extremely restricted performance. This has, perhaps at least in part, resulted from radio practice for surface installations in free space. There, there are usually severe and detailed regulations concerning what frequencies one can use for radio communication, and in most countries and environments there is strong competition for use of available frequencies.